Conventional ultrasound imaging system comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred beam direction and is focused at a selected range along the beam. Multiple firings may be used at various depths or different positions in the image to acquire a full two dimensional data set representing the desired anatomical information along a multiplicity of scan lines. The beamforming parameters of each of the firings (or transmitted beams) may be varied to provide a change in the position of focus, depth of field or the shading (or apodization) function. Similarly, the beam forming parameters can be changed for the received beam. A dynamic receive beam is typically used for the reception where the delay focus is continuously changed as different data are received from different depths. However, during the transmission a beam of ultrasound energy with a specific focal position is transmitted. Typically multiple beams are transmitted along the same direction with different focal lengths for improved resolution. Multiple transmit and reception beams are used in a plane to construct a two dimensional image.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element. The reflected ultrasound is sampled from the focal zones of two or more transmit beams each focused at different depths along the same scan line. In most recent ultrasound imaging systems the received signal is dynamically focused as signals from different depths are received. For each steering angle, the sampled data from contiguous focal zones is acquired and then spliced to make one vector or A-line. A multiplicity of transmit vectors, one beam for each focal point, are used, along with interpolated data values, are used to collect all the image information which are displayed on the monitor to form a full image frame. This information is displayed on a pixel by pixel basis.
Such scanning comprises a series of measurements in which the steered or non-steered beams of ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected, or backscattered, ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along a scan line. Multiple reception beams can be formed for a single transmit beam for improved frame rate. For example, for a single transmit beam two reception beams on either side of transmit beam can be formed simultaneously using parallel beamforming or alternatively using high-speed a multiplexed beamforming which would process both beams simultaneously. The receiver is dynamically focused at a succession of ranges or depths along the scan line as the reflected ultrasonic waves are received.
Referring to FIG. 1, the ultrasonic imaging system incorporating the invention includes a transducer array 10 comprised of a plurality of separately driven transducer elements 12, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 22. The ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer element 12 and applied separately to a receiver 24 through a set of transmit/receive (T/R) switches 26. Transmitter 22, receiver 24 and switches 26 are operated under control of a digital controller 28 responsive to commands by a human operator. A complete scan is performed by acquiring a series of echoes in which switches 26 are set to their transmit position, transmitter 22 is gated ON momentarily to energize each transducer element 12, switches 26 are then set to their receive position, and the subsequent echo signals detected by each transducer element 12 are applied to receiver 24, which combines, or beamform, the separate echo signals from each transducer element to produce a single echo signal which is used to produce a line in an image on a display monitor 30.
Transmitter 22 drives transducer array 10 such that the produced beam of ultrasonic energy is directed, or steered, along a specific steering angle. To accomplish this, transmitter 22 imparts a time delay T.sub.i to the respective pulsed waveforms 34 that are applied to successive transducer elements 12. By adjusting the time delays T.sub.i appropriately in a conventional manner, the ultrasonic beam can be directed away from the normal to the plane of transducer array 36, by an angle .theta. and/or focused at a fixed range R. A sector scan is performed by progressively changing the time delays T.sub.i in successive excitations. The angle .theta. is thus changed in increments to steer the transmitted beam in a succession of directions.
The echo signals are produced by each burst of ultrasonic energy, reflect from objects located at successive ranges along the ultrasonic beam. The echo signals are sensed separately by each transducer element 12 and a sample of the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range. Due to the differences in the propagation paths between a reflecting point P and each transducer element 12, however, these echo signals will not be detected simultaneously and their amplitudes will not be equal. Receiver 24 amplifies the separate echo signals, imparts the proper time delay to each, and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from point P located at range R along the ultrasonic beam oriented at the angle .theta.. Demodulation can occur either before or after the individual received signals are summed together.
To simultaneously sum the electrical signals produced by the echoes impinging on each transducer element 12, time delays are introduced into each separate transducer channel 110 of receiver 24 (see FIG. 2). The beam time delays for reception are delays (T.sub.i) which are applied in a similar manner as the transmission delays described above. However, the time delay of each receiver channel is continuously changing during reception of the echo to provide dynamic focusing of the received beam at the range R from which the echo signal emanates.
Under the direction of digital controller 28, receiver 24 provides delays during the scan such that steering of receiver 24 tracks the direction .theta. of the beam steered by transmitter 22 and samples the echo signals at a succession of ranges R and provides the proper delays and phase shifts to dynamically focus at points P along the beam. Thus, each transmission of an ultrasonic pulse waveform results in the acquisition of a series of data points which represent the amount of reflected sound at points in the focal zone of the transmit beam.
Referring to FIG. 1, scan converter/interpolator 32 receives the series of data points produced by receiver 24 and converts the data into the desired image for display. In particular, the scan converter converts the acoustic image data from polar coordinate (R.sub.---- .theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on display monitor 30, which images the time-varying amplitude of the envelope of the signal as a gray scale.
Referring to FIG. 2, a conventional receiver 24 comprises three sections: a time-gain control section 100, a receive beamforming section 38 and a processor 102. Time-gain control (TGC) section 100 includes a respective amplifier 105 for each of the receiver channels 110 and a time-gain control circuit 106. The input of each amplifier 105 is connected to a respective one of transducer elements 12 to amplify the echo signal which it receives. The amount of amplification provided by amplifiers 105 is controlled through a control line 107 that is driven by TGC controller 106. The TGC is a combination of the potentiometers 108 which are set by the operator together with a constant gain profile, programmed into the controller, which compensates for tissue attenuation and diffraction gain variation in the image.
The receive beamforming section 38 of receiver 24 includes separate receiver channels 110. Each receiver channel 110 receives the analog echo signal from one of amplifiers 105 at an input 111. Each received signal is delayed before being summed at the summing point 114 and 115. This delay provides the dynamic focusing which is essential for high resolution imaging. The summed signals indicate the magnitude and phase of the echo signal reflected from a point P located at range R on the steered beam (.theta.). Each amplified signal is conveyed as a pair of quadrature signals in the respective receiver channel, where the phases of the mixing reference frequency differ by 90.degree.. Alternatively the quadrature signals can be produced using the Hilbert transform. A signal processor 120 receives the beam samples from summing points 114 and 115 and produces an output 121 to scan converter 32 (see FIG. 1). The signal processor 120 sums the square of the I and Q signals before taking the square root of this signal. This produces the envelope detected or demodulated image signal. Alternatively, the demodulation can be performed after the individual received signals are summed. The signal processor 120 comprises an envelope detector for forming the envelope of the complex signals (I and Q), at which point the phase information is lost.
The axial resolution of an ultrasound imaging system of the foregoing type is primarily determined by the finite bandwidth of the transducer. In accordance with conventional ultrasound imaging methods, the highest possible resolution is obtained by means of an impulse excitation which utilizes the entire available bandwidth of the transducer. Unfortunately, the available energy in an impulse excitation is low, which results in poor sensitivity. In order to compensate for this, a larger driver pulse can be used. However, there are a number of factors which limit the amount of peak-to-peak voltage which can be applied to a transducer. These limitations are brought about by the finite peak-to-peak voltage available from the driver electronics, the breakdown voltage of the piezoceramic material and the possibility of depoling of the piezoceramic or piezoelectric material, the need for high-voltage driver stages and regulatory limits on the peak pressure to which a patient can be exposed. Furthermore, under impulse excitation the bandwidth of the emitted pulse is limited to the transducer bandwidth. The increased bandwidth would result in an improved spatial resolution, improved contrast resolution and an improved depth of field.